The aviation industry relies upon numerous navigation aids in order to safely take off, navigate enroute, and land aircraft. Among these, the instrument landing system (ILS) is the internationally accepted and standardized navigation aid for landing aircraft at properly equipped airports. GPS, however, is increasingly being accepted as an alternative to traditional navigation aids, including even ILS.
Essentially, GPS is a space based radio positioning network for providing users equipped with suitable receivers highly accurate position, velocity, and time (PVT) information. Developed by the United States Department of Defense (DOD), the space based portion of GPS comprises a constellation of GPS satellites in non-geosynchronous orbits around the earth.
FIG. 1 shows the constellation 100 of GPS satellites 101 in orbit. The GPS satellites 101 are located in six orbital planes 102 with four of the GPS satellites 101 in each plane, plus a number of on orbit spare satellites (not shown) for redundancy. The orbital planes 102 of the GPS satellites 101 have an inclination of 55 degrees relative to the equator and an altitude of approximately 20,200 km (10,900 miles) and typically complete an orbit in about 12 hours. This positions each of the GPS satellites 101 in such a manner that a minimum of five of the GPS satellites 101 are normally observable (above the horizon) by a user anywhere on earth at any given time.
The orbiting GPS satellites 101 each broadcast spread spectrum microwave signals encoded with positioning data. The signals are broadcast on two frequencies, L1 at 1575.42 and L2 at 1227.60, with the positioning data modulated using bi-phase shift keying techniques. A user receives the signals with a GPS receiver. The GPS receiver is adapted to demodulate the positioning data contained in the signals. Using the positioning data, the GPS receiver is able to determine the distance between the GPS receiver and a corresponding transmitting GPS satellite. By receiving signals from several of the GPS satellites 101 and determining their corresponding range, the GPS receiver is able to determine its position and velocity with a greater accuracy than conventional radio navaids.
Applications of GPS to aircraft navigation are currently partitioned into two main areas. The first area includes the en route, terminal, and non-precision approach phases of flight. The second area is the precision approach phase of flight. This is a natural partition for both historical and practical reasons. The GPS signals commonly available to civilian users are referred to as the standard positioning service (SPS). The accuracy of SPS is specified by DOD to be within 100 meters horizontal positioning accuracy 95% of the time and 300 meters 99.99% of the time. The 100 meter accuracy specification currently is sufficient, i.e., at least as accurate or better than current approved navigation systems, for all phases of flight down to and including non-precision approaches. SPS, however, is not sufficiently accurate for vertical guidance in the precision approach phase of flight.
The lateral and vertical navigation sensor accuracies for precision approach traditionally have been based on three categories of approach: Category I (CAT-I), Category II (CAT-II), or Category III (CAT-III). A precision approach is where an aircraft relies primarily upon instruments for landing, due to bad weather or other constraints. The operational definitions of these categories are based on visibility (runway visual range) and landing decision height. Category III has the most stringent requirements, including very stringent equipment redundancies, lateral guidance in roll out for Category IIIb and Category IIIc, and other requirements. Table 1 below summarizes the requirements specified by the traditional categories.
TABLE 1 ______________________________________ Traditional categories of precision approach Category Runway visual range Decision height ______________________________________ CAT-I 1800-2400 ft 200 ft CAT-II 1200 ft 100 ft CAT-IIIa &gt;700 ft &lt;100 ft CAT-IIIb 150-700 ft &lt;50 ft CAT-IIIc &lt;150 ft 0 ft ______________________________________
FIG. 2A shows a schematic diagram a down view of an airport runway 20 relative to a flight path 21 of an aircraft 22, and FIG. 2B shows a corresponding side view. The aircraft 22 has executed a CAT-II approach. The aircraft 22 follows a glide slope as it approaches the runway 20, creating a flight path 21. A decision height is that height above the runway at which aircraft 22 must declare a missed approach if the runway is not yet in view. In the present example, a CAT-II approach, required RVR 24 is 1200 ft and the required decision height 25 is 100 ft above runway 20. Thus, CAT-IIIc allows flight right down to the runway surface (decision height of 0 ft) with potentially zero RVR. As can be expected, requirements for CAT-IIIc approaches are very stringent.
Before GPS can be used by the aviation community for precision approaches, GPS aviation electronics (avionics) need to be certified for these most demanding and flight critical phases of flight. Aircraft flying precision approaches utilize the well known ILS technology. ILS uses a ground based azimuth transmitter (localizer) and a ground based elevation transmitter (glide slope) which define a precision approach flight path to be followed. By employing ILS receivers, a properly equipped aircraft is able to fly down the ILS flight path to land on the runway.
Current CAT-III ILS systems used in newer aircraft employ dual ILS receivers on board the aircraft for added redundancy and fault detection. The aircraft's autopilot compares the outputs of these two receivers, and if they disagree by more than a certain predetermined amount, a fault is declared, and the aircraft must execute a missed approach. The CAT-III ILS systems are routinely accurate to within 2 ft in the vertical dimension at 100 feet above the runway surface. They are required to work under extreme weather conditions and at life-critical levels of performance (for example, the probability of a missed detection of failure not exceeding 5.times.10.sup.-9). This is the standard at which CAT-III GPS systems are expected to perform.
There are several prior art methods being considered for employing GPS for CAT-III auto landings. One method involves using differential GPS (DGPS) techniques to improve the accuracy and fault detection capability of SPS. DGPS involves placing a local area augmentation system DGPS transmitter near the airport. The transmitter broadcasts DGPS corrections and integrity data to nearby aircraft which use the data to determine their accurate DPGS positions. Although the DGPS positions tend to be sufficiently accurate in the horizontal dimension, they have much less margin in the vertical dimension. In addition, the code phase measurements used in typical prior art DGPS systems tend to be noisy in comparison to ILS determined measurements. Thus, even though DGPS is potentially more accurate overall than ILS, the noise characteristics are such, and the ILS comparison threshold is set so low, that two GPS receivers will disagree by enough that a fault would get declared by the autopilot on approximately 50% of the approaches. This is because ILS receivers exhibit very low noise, even though the accuracy of the signal is usually inferior to DGPS. This relationship is illustrated below.
FIG. 2C shows a comparison of the relative accuracy and error characteristics of ILS versus DGPS. Graph 30 shows the angular error of two ILS receivers (e.g., ILS #1 and ILS #2). The vertical direction of graph 30 represents angular error and the horizontal direction represents time. In graph 30, ILS #1 and ILS #2 both begin the approach at zero angular error (e.g., on the left side). As time progresses, as the aircraft proceeds down the approach path toward a runway threshold, the angular error of ILS #1 and ILS #2 vary from zero angular error. This variance, however, is such that ILS #1 and ILS #2 remain within a predetermined upper and lower alarm limit.
Graph 31 shows the total angular distance between ILS #1 and ILS #2, corresponding to graph 30. Thus, even though ILS #1 and ILS #2 vary from zero angular error, the difference between them does not exceed the upper or lower alarm limit (e.g., which would trigger an autopilot fault).
Graph 32 is similar to graph 30 except that, instead of two ILS receivers, two DGPS receivers (e.g., DGPS #1 and DGPS #2) are plotted. Graph 32 shows how even though GPS is much more accurate than ILS when the aircraft is far from the runway threshold, the nature of DGPS is such that, as the aircraft approaches the runway threshold, the angular error of DGPS #1 and DGPS #2 increases. In addition, the angular error of DGPS #1 and DGPS #2 vary independently with respect to each other.
Graph 33 shows the total angular distance between DGPS #1 and DGPS #2. Thus, since the angular error of DGPS #1 and DGPS #2 increases as the aircraft approaches the runway threshold and since their angular error varies independently, the total angular error exceeds the upper and lower alarm limits, causing alarm conditions. Consequently, the autopilot declares a fault and the approach is aborted.
One solution to this problem being considered is the use of carrier phase DGPS techniques. One such prior art technique is the Integrity Beacon Landing System (IBLS) technique. Using IBLS, low power pseudolite transmitters are located on either side of the precision approach flight path to a runway. The pseudolite transmitters broadcast a signal, along with a signal being broadcast from a conventional DGPS transmitter located near the runway, such that an aircraft flying along the precision approach flight path is capable of tracking enough signal sources (GPS satellites and pseudolites) to unambiguously determine a carrier phase DGPS position accurate to within centimeters. Thus, the carrier phase DGPS position is far more accurate than the CAT-III ILS noise thresholds such that two GPS receivers will easily agree by enough margin that a fault will not be declared. The problem with this solution is that it requires expensive construction and maintenance of pseudolite transmitters off of the airport property. In many instances, this can be very expensive.
While in principle, it should merely be a matter of resetting the autopilot threshold, it turns out that doing this would be extremely costly due to enormous amount of testing required for certifying CAT-III standards. The aviation industry wants GPS landing equipment to look just like ILS, behave like ILS, and interface with other aircraft systems just like ILS. For example, aviation industry standard autopilots are designed to interface with dual ILS systems. An ideal solution would interface with industry standard autopilots, use similar hardware and software interfaces, and be subject to similar comparison between dual GPS avionics receivers and perform at a similar level of integrity (e.g., no noise caused autopilot fault declarations).
Thus what is required is a method and system for employing GPS for CAT-III auto landings which do not require locating equipment off of the airport property. What is further required is a method and system for employing GPS which will interface with existing autopilot systems currently installed in the fleet. Such a method and system should not cause an inordinate amount of autopilot fault declarations. What is further desired is a method and system which provides the benefits of GPS while retaining the stringent CAT-III certification standards of prior art dual ILS systems.